Epoxy resins are used as an adhesive, a molding material, a coating material, a photoresist material, a color developer, etc. In addition, epoxy resins are widely used in the electric/electronic fields such as a semiconductor sealing material and an insulating material for a printed wiring board from the standpoint of good heat resistance, moisture resistance, and the like of the resulting cured products.
Among these various applications, in the field of printed wiring boards, with a trend of a reduction in the size and an increase in the performance of electronic devices, a trend of realizing a high density by reducing the wiring pitch of semiconductor devices has been significant. As a semiconductor packaging method for meeting this trend, a flip-chip connection method in which a semiconductor device is bonded to a substrate with solder balls is widely used. This flip-chip connection method is a semiconductor packaging method using a so-called reflow process in which solder balls are arranged between a wiring board and a semiconductor, and fusion bonding is performed by heating the whole components. Accordingly, during the solder reflow, the wiring board itself is exposed to a high-heat environment, and a large stress is generated, by thermal contraction of the wiring board, in the solder balls connecting the wiring board to the semiconductor, which may result in connection failure of wiring. Therefore, a material having a low coefficient of thermal expansion has been desired for an insulating material used as a printed wiring board.
In addition, recently, lead-free high-melting point solder has been widely used because of, for example, regulations associated with environmental issues. This lead-free solder is used at a temperature about 20° C. to 40° C. higher than the temperature used for existing eutectic solder. Thus, a heat resistance higher than ever before is required for curable resin compositions.
As described above, high heat resistance and low thermal expansion have been desired for insulating materials for printed wiring boards. As an epoxy resin material that can meet such requirements, for example, a tetrafunctional naphthalene epoxy resin represented by the structural formula below is known (refer to PTL 1 below).

The above tetrafunctional naphthalene epoxy resin has a crosslinking density higher than that of general phenol novolac-type epoxy resins, and thus good low thermal linear expansion and heat resistance are exhibited in cured products of the epoxy resin. However, recently, higher performance has been required, and it has been necessary to achieve a further improvement. Furthermore, since the tetrafunctional naphthalene epoxy resin has low solubility in solvents that are generally used in the production of printed wiring boards, characteristics of resulting cured products are not sufficiently exhibited.